Method of electroplating a bright adherent chromium coating onto cast-iron



United States Patent Ofiice 3,421,986 Patented Jan. 14, 1969 3,421,986 METHOD OF ELECTROPLATING A BRIGHT ADHERENT CHROMIUM COATING ONTO CAST-IRON Wolfram Ruff, Frankfurt am Main, and Heinz Boucher,

Neu Isenburg, Germany, assignors to Alfred Teves Maschinenund Armaturenfabrik KG., Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed July 23, 1965, Ser. No. 475,044 Claims priority, application Glermany, Oct. 28, 1964,

US. Cl. 204-36 4 Claims Int. 01. C23b 5/50 ABSTRACT OF THE DISCLOSURE This invention relates to chromium plating and, more particularly, to the electroplating of soft chromium on metal substrates.

Plates of soft chromium provide excellent corrosion resistance and in addition provide attractive surfaces that lend themselves to polishing processes that yield scratchfree and fissureless coatings possessing high lustre.

Soft chromium plating is the deposition of chromium upon metal substrates (e.g. cast iron) in a coating having a hardness ranging from 50 to 800 DPH Vickers hardness units when measured by the Vickers Diamond-Point Hardness Tester under a 500 gram load. Such hardness has been recognized by industry as yielding scratchless finishes of high lustre by conventional polishing processes. The soft chromium as deposited from the plating solutions is matte-gray.

In the past, by conventional chromium plating processes, soft chromium layers have been deposited upon such substrates but, due to the poor plating efliciencies of these conventional electrolytes, such plating is expensive. The conventional electrolytes have had, for example, plating efliciencies ranging up to 28%. In order to deposit sufiicient thicknesses of chromium upon the substrates, inordinately long plating times are required.

In the past various attempts to increase the plating efliciencies in order to shorten the deposition times have proven impractical because of the deleterious effects of the diflierent processes upon the nature of the deposited chromium.

Some of such earlier processes have evidenced poor throwing power with resultant uneven deposition of the chromium upon irregular objects. Other processes often resulted in chromium layers with poor adherence to the substrate. Still other processes did not deposit the proper type of chromium, i.e. the layer was often too hard to be polished by industrial polishing methods.

Recently attempts have been made to improve the plating efiiciencies of the plating baths by using alkalimetal compounds as additives to the plating electrolytes. Baths containing sodium tetrachromate have been proposed. Such baths are claimed to give higher than conventional plating efiiciencies and consequently higher rates of chromium deposition. Plating efliciencies as high as 37% have been claimed for these baths by operating at current densities exceeding 1000 amperes per dm. (square decimeters). The temperatures of the electrolyte in such processes must be maintained below 40 C. as this is the upper limit for the stability of sodium tetrachromate. The sodium tetrachromate is produced by the addition of sodium hydroxide and sodium chromate to a chromic acid solution where the molecular (molar) ratio of Na O and to CrO ranges between 1:4 and 1:6.

It is an object of this invention to provide a process for the deposition of soft chromium at high efiiciencies.

It is a further object of this invention to provide welladhering and uniform soft-chromium deposits even when the deposits are relatively thick.

It is a further object of this invention to provide a chromium-plating process and electrolyte suitable for the production of plated layers of soft chromium that, by comparison with prior-art plated layers have reduced brittleness, increased ductility, minimal cracking tendencies, improved corrosion resistance and excellent ability to be polished.

Another object of this invention is to provide a process and electrolytic bath for the purposes described which are economical to use, easy to control, simple to prepare and which contains relatively inexpensive ingredients.

These and other objects of this invention can, surprisingly, be realized by the electrodepositing of soft chromium from alkali-free baths (e.g. containing no Na O/ NaOH or alkali-metal ions), at temperatures in the range 20 to 40 C., but preferably between 25 to 35 C., and at current densities ranging from 60 to 120 amperes per dm. when the electrolyte or bath contains between substantially 0.45 and 0.75 (by weight) of SO ion (as free ions) and between substantially 200 to 400 grams of CrO per liter.

When operating under the process set forth herein, plating efiiciencies of 40 to 60% are realized. As a result of such high plating efficiencies it is possible to plate soft-chromium deposits at an accretion rate of more than 70 per hour.

The various limits set forth for this invention are characteristic and critical. At a temperature of 20 C., within the limits of this process, the plating efficiency was found to be over 57%, but at 56 C., which is outside the limits, the plating efficiency dropped to 17%.

Similarly, when the CrO content of the bath varied either above 400 grams/liter or below 200 grams/liter, the efficiency dropped drastically. An S0 ion content of less than 0.4% (by weight) decreased the electrochemical efficiency and therefore the rate of chromium deposition. When the 80,-- ion concentration of the electrolyte exceeded 0.9%, the coating capacity of the bath was also reduced and the throwing power was very poor.

It has been known that thick deposits of soft chromium (over 25,11.) do not adhere too well to metallic substrates particularly when these substrates are, for instance, composed of cast iron. According to a further aspect of this invention it is now possible to obtain good adherence of soft chromium deposits even to cast iron or other limitedly adhereable substrates by the process of this invention. This feature of the invention is based on covering the substrate first with a 3 to 8 intermediate layer of hard chromium and then proceeding according to the process as previously set forth.

The hard-chromium intermediate layers, according to this invention, are deposited at temperatures in the range 45 to C. from conventional baths for deposition of hard chromium (i.e. containing in excess of, say, 1% free sulfate ion and about 250 grams CrO /liter). It is believed that the higher bath temperatures required for the deposition of the hard chromium causes activation results in the excellent adherence of the electrolytically 3 deposited hard chromium which can have a Vickers hardness of 1000 to 1100 DPH units. Under the operating temperatures for the deposition of soft chromium according to this invention, it appears that base metals such as cast iron are insufficiently activated to provide proper adherence of the deposited layer.

The hard-chromium layer can be deposited from electrolytes either containing fluosilicic acid or free from fluosilicic acid as described by P. Morisset et al. in Hard and Decorative Chromium Plating, Centre dInformation du Chrome Dur, 1961, Paris VIII. When the electrolyte does not contain this acid the deposited hard-chromium layer should be subjected either to a pickling in sulfuric acid or to anodization in chromic acid. Where the electrolyte does contain the fluosilicic acid the soft chromium layer can be directly deposited upon the hard chromium layer when the body emerges from the hard-chromium plating bath. Utilizing such an intermediate hard chromium layer permits deposition of soft chromium upon the cast iron base metal in layers of more than 60p. in thickness with excellent adhesion of the layers to base metal.

The invention will be more particularly described in the example which follows. This example is merely illustrative of the parameters of this invention and sets forth an operative process within the ambit of this invention.

EXAMPLE Using usual techniques as described in the aforementioned text by P. Morrisset et al., a 6-micron coating of hard chromium (Vickers DPH=1000-1100) is electrodesposited upon a cast-iron body and treated with sulfuric acid or anodically oxidized as set forth above. After such treatment or after direct removal from a fluosilicic acid hard-chromium plating bath if one is used, the casting is immersed in a soft-chromium plating bath constituted of an alkali-free aqueous solution of 318 grams/liter CrO 32.7 grams/liter C12O3, and 0.67% free sulfate ion, and electrodeposition carried out at a current density of 71.5 amperes/decimeter against a lead electrode to 10 volts) at a temperature of C. With a current efficiency in excess of about 50% to yield a soft-chromium coating of a thickness of about 25 microns. The coating was free from fissures and had a hardness (Vickers DPH) of about 600. The coating was rinsed and polished readily by conventional techniques to a high luster. It was found to be strongly adherent, uniform and wear-resistant. When the temperature was reduced to 20 C., the current (plating) efficiency was about 57% although lower temperatures rendered the process uneconomical because of increased plating time and difficulties of controlling the uniformity of plating. A temperature of 56 C. gave a reduced plating efficiency of only 17%.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being considered within the spirit and scope of the appended claims.

We claim:

1. A method of chromium-plating a cast-iron body, comprising the steps of:

(a) electroplating a relatively thin hard-chromium layer onto said body at a temperature above 45 C. in a hard-chromium plating bath to a. thickness of 3 to 8 microns;

(b) treating the hard-chromium layer with sulfuric acid;

(c) depositing a layer of soft chromium upon the treated hard-chromium layer of step (b) at a current density of to amperes per dm. in an alkalifree aqueous electrolyte containing 0.45 to 0.75% free-sulfate ion and 200 to 400 g. of chromium trioxide per liter at a temperature between 20 C. and 40 C. to a thickness of at least 25 microns; and

(d) polishing the soft chromium layer deposited in step (c).

2. The method defined in claim 1 wherein said soft chromium layer has a thickness of at least 60 microns, a hardness of about 600 DPH on the Vickers scale, the soft chromium plating of step (0) being carried out at a temperature of 25 C. to 35 C.

3. A method of chromium-plating a cast-iron body, comprising the steps of:

(a) electroplating a relatively thin hard-chromium layer onto said body at a temperature above 45 C. in a fluosilicic acid hard-chromium plating bath to a thickness of 3 to 8 microns;

(b) depositing a layer of soft chromium upon the hardchromium layer at a current density of 60 to 120 amperes per drn. in an alkali-free aqueous electrolyte containing 0.45 to 0.75% free-sulfate ion and 200 to 400 g. of chromium trioxide per liter at a temperature between 20 C. and 40 C. to a thickness of at least 25 microns; and

(c) polishing the soft chromium layer deposited in step (b).

4. The method defined in claim 3 wherein said soft chromium layer has a thickness of at least 60 microns, a hardness of about 600 DPH on the Vickers scale, the soft chromium plating of step (b) being carried out at a temperature of 25 C. to 35 C.

References Cited UNITED STATES PATENTS 2,118,956 5/1938 Wagner 20432 2,846,380 8/1958 Brown 20451 2,856,334 10/1958 Topelian 20441 XR 3,337,430 8/1967 Johnson 20451 FOREIGN PATENTS 503,903 4/ 1939 Great Britain.

ROBERT K. MIHALEK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

US. 01. X.R. 204-29, 41, 51 

